1. Field of the Invention
The present invention relates to a stepping motor, and more particularly, to a cylindrical stepping motor.
2. Description of the Related Art
As a first conventional example, a stepping motor having a small diameter centering on a rotary shaft and an increased output has been proposed (for example, see Japanese Patent Application Laid-Open No. 9-331666, corresponding to U.S. Pat. No. 5,831,356).
FIG. 9 is an exploded perspective view of the stepping motor of the first conventional example, and FIG. 10 is a longitudinal cross-sectional view of the stepping motor shown in FIG. 9.
In FIGS. 9 and 10, the stepping motor of the first conventional example includes a first stator 204 and a second stator 205 made of a soft magnetic material, a connection ring 207 made of a nonmagnetic material, an output shaft 206, and a rotor 201 made of a permanent magnetic material. The first stator 204 and the second stator 205 are opposed to each other at a predetermined interval in the axial direction of the motor. The connection ring 207 keeps the first stator 204 and the second stator 205 having the predetermined interval therebetween. The output shaft 206 is rotatably supported by a bearing part 204E of the first stator 204 and a bearing part 205E of the second stator 205. The rotor 201 is fixed to the output shaft 206 and is divided into four pieces in the circumferential direction, which are alternately magnetized into different poles.
The first stator 204 has a top end part having a comb-tooth shape and includes first outer magnetic poles 204A and 204B and first inner magnetic poles 204C and 204D. The first outer magnetic poles 204A and 204B are opposed to the outer circumferential surface of the rotor 201 at a predetermined space. The first inner magnetic poles 204C and 204D are opposed to the inner circumferential surface of the rotor 201 at a predetermined space. The second stator 205 includes second outer magnetic poles 205A and 205B and second inner magnetic poles 205C and 205D. The second outer magnetic poles 205A and 205B are opposed to the outer circumferential surface of the rotor 201 at a predetermined space. The second inner magnetic poles 205C and 205D are opposed to the inner circumferential surface of the rotor 201 at a predetermined space.
On the first inner magnetic poles 204C and 204D, a first coil 202 for exciting the first stator 204 is wound adjacent to the rotor 201 in the axial direction of the motor. On the second inner magnetic poles 205C and 205D, a second coil 203 for exciting the second stator 205 is wound adjacent to the rotor 201 in the axial direction of the motor.
In the stepping motor configured as described above, the rotor 201 is rotated by switching the energizing direction to the first coil 202 and the second coil 203 to switch each magnetic polarity of the first outer magnetic poles 204A and 204B, the first inner magnetic poles 204C and 204D, the second outer magnetic poles 205A and 205B, and the second inner magnetic poles 205C and 205D.
In the stepping motor, a magnetic flux generated by energizing the coil flows from the outer magnetic pole to the inner magnetic pole opposed to the outer magnetic pole, or from the inner magnetic pole to the outer magnetic pole opposed to the inner magnetic pole, to thereby effectively act on the cylindrical magnet positioned between the outer magnetic pole and the inner magnetic pole. Further, since the interval between the outer magnetic pole and the inner magnetic pole can be made into the thickness of the cylindrical magnet, resistance of a magnetic circuit composed of the outer magnetic pole and the inner magnetic pole can be decreased. Thus, it is possible to generate a large magnetic flux with a little current, to thereby increase the output.
Further, as a second conventional example configured by improving the above-described stepping motor, the following stepping motor has been proposed. That is, in the stepping motor, an inner magnetic pole is made to have a cylindrical shape, an output shaft inserted into an inner diameter part of the inner magnetic pole is made of a soft magnetic material, and a bearing part mounted on a stator to rotatably keep the output shaft is made of a nonmagnetic material (see Japanese Patent Application Laid-Open No. 10-229670). According to this stepping motor, the output of the motor can be increased by using the output shaft as a magnetic circuit. Further, by making the bearing part of a nonmagnetic material, the magnetic attraction between the stator and the output shaft can be prevented when increasing the output of the motor.
Further, as a third conventional example configured by improving the above-described stepping motor of the first conventional example, a stepping motor in which a rotary shaft of the motor is urged in the axial direction has been proposed (see Japanese Patent Application Laid-Open No. 2000-287434, corresponding to U.S. Pat. No. 6,255,749).
FIG. 11 is a longitudinal cross-sectional view of the stepping motor of the third conventional example.
In FIG. 11, the stepping motor of the third conventional example includes a first stator 318, a second stator 319, a connection ring 320, a frame 323, an output shaft 307, and a cylindrical rotor 301. The first stator 318 and the second stator 319 are made of a soft magnetic material and are opposed to each other at a predetermined interval in the axial direction of the motor. The connection ring 320 connects the first stator 318 with the second stator 319. The frame 323 is fixed to the second stator 319. The output shaft 307 is rotatably supported by a bearing part 325 of the first stator 318 and a top end bearing part 324 of the frame 323. The rotor 301 is made of a permanent magnet and is fixed to the output shaft 307 by press fitting.
The first stator 318 has a top end part having a comb-tooth shape and includes an outer cylinder 318a, an inner cylinder 318b, and a first auxiliary stator 321. The outer cylinder 318a is opposed to an outer circumferential surface of the rotor 301 at a predetermined interval, to thereby constitute a first outer magnetic pole. The inner cylinder 318b is opposed to an inner circumferential surface of the rotor 301 at a predetermined interval. The first auxiliary stator 321 constitutes a first inner magnetic pole in association with the inner cylinder 318b. The second stator 319 has a top end part having a comb-tooth shape and includes an outer cylinder 319a, an inner cylinder 319b, and a second auxiliary stator 322. The outer cylinder 319a is opposed to the outer circumferential surface of the rotor 301 at a predetermined interval, to thereby constitute a second outer magnetic pole. The inner cylinder 319b is opposed to the inner circumferential surface of the rotor 301 at a predetermined interval. The second auxiliary stator 322 constitutes a second inner magnetic pole in association with the inner cylinder 319b. 
On the first inner cylinder 318b, a first coil 302 for exciting the first stator 318 is wound adjacent to the rotor 301 in the axial direction of the motor. On the second inner cylinder 319b, a second coil 303 for exciting the second stator 319 is wound adjacent to the rotor 301 in the axial direction of the motor.
A lead screw part 307a is formed on the output shaft 307 and is engaged with a female screw (not shown) to linearly move the female screw when the output shaft 307 is rotated.
The first inner cylinder 318b of the first stator 318 contains a slide member 326 housed therein, a cover 328 fixed to an end part thereof, and a compression coil spring 327. The compression coil spring 327 urges the output shaft 307 towards the top end bearing part 324 through the slide member 326. Thus, concerning the position of the output shaft 307 in the axial direction, the output shaft 307 is urged towards the top end bearing part 324 by the compression coil spring 327. Thereby, the position in the axial direction of the output shaft 307 is defined while removing a hysteresis difference generated by a rotating position of the output shaft 307. In this case, since an urging unit composed of the compression coil spring 327 and the slide member 326 is housed inside the first inner cylinder 318b, the stepping motor can be configured in a compact manner.
However, as for the stepping motors proposed in the above-described first conventional example to the third conventional example, these motors are required to have a predetermined clearance between the inner diameter of the magnet and the inner magnetic pole opposed thereto, and controlling this clearance increases cost in the production process. Further, as for the shape of the stator, the cylindrical inner magnetic pole and outer magnetic pole are required, and it is hard to integrally construct these poles in the process of manufacturing parts. Furthermore, if these poles are manufactured separately and, thereafter, integrally assembled, the number of parts is increased, thus resulting in a cost increase.
Further, as for the stepping motor proposed in the above-described second conventional example, a magnetic flux generated by energizing the first coil affects the second coil, the second outer magnetic pole, and the second inner magnetic pole through the output shaft made of a soft magnetic material. A magnetic flux generated by energizing the second coil affects the first coil, the first outer magnetic pole, and the first inner magnetic pole through the output shaft made of a soft magnetic material. Thereby, the rotation of the stepping motor becomes unstable.
Furthermore, as for the stepping motor proposed in the above-described third conventional example, since an urging unit is provided inside the inner magnetic pole, a space in the diameter direction of the coil is small as compared with the stepping motor proposed in the above-described first conventional example, so that the output of the stepping motor is decreased. Further, if the diameter of the motor is further reduced, it is hard to provide the urging unit. That is, if the outer diameter of the motor is reduced while keeping the minimum size of the urging unit, the width in the diameter direction of the motor must be reduced, so that an output torque of the stepping motor is decreased.